Organic light emitting devices (i.e., OLEDs) produce light emission upon application of current. See, for example, Tang and VanSlyke, Appl. Phys. Lett., 51, (1987), p. 913-915; Burroughs, et al., Nature, 347, (1990), p. 539-541. In a simplest form, OLEDs are comprised of one or more thin layers of organic light emissive materials sandwiched between two conductors (i.e., electrodes). An applied potential across the electrodes injects electrons and holes from the cathode and anode, respectively, ultimately producing an excited state light emissive material. Light is produced by photon emission by the excited light emissive material as it returns to the ground state. Preferably, the barrier to electron or hole injection at both electrode interfaces should be kept low to reduce power consumption.
A wide variety of organic materials have been used to transport charge (electron or hole) from the electrode to the light emissive material. However, the requirements for the cathode and anode are such that relatively few choices are available. Specifically, it is important that the work functions of the electrodes are well matched to the appropriate energy levels in the organic charge transport materials.
Conventional efforts have focused on OLEDs using tris(8-hydroxyquinoline) aluminum(III) complex (Alq3) as the light emissive and electron transport layer, and a triarylamine compound as the hole transport medium. The advantage of this approach lies in part in the emission of light at or near the interface between the two organic layers.
Without being bound by any theory, it is believed that a low barrier for injection should be provided when the Fermi energy of the cathode is closely matched to the lowest unoccupied molecular orbital (LUMO) energy of the light emitting (or electron transport) layer. To this end, low work function (Φ) metals such as magnesium, calcium, and aluminum, or their alloys with silver are the most commonly used materials. See, for example, Tang et al., Appl. Phys. Lett., 1987, 51, 913; Burrows et al., Appl. Phys. Lett., 1994, 64, 2285; and Matsumura et al., J. Appl. Phys., 1996, 79, 264.
Recently, thin layers of insulators and wide band gap semiconductors between the conducting cathode and the active organic material have been utilized. Specifically, inorganic insulators such as LiF, Li2O, MgF2, and MgO under aluminum metal have been widely used, (see Hung et al., Appl. Phys. Lett., 1997, 70, 152; Jabbour et al., Appl. Phys. Lett., 1997, 71, 1762; Matsumura et al., Appl. Phys. Lett., 1998, 73, 2872; and Lee, Synth. Met., 1997, 91, 125) and copper phthalocyanine (CuPc) has been paired with the high work function conducting metal oxide indium tin oxide (ITO) (Parthasarathy et al., Appl. Phys. Lett., 1998, 72, 2138) to produce a reasonably efficient transparent OLED. Doping of lithium metal into Alq3 by codeposition has also been reported. Kido et al., Appl. Phys. Lett., 1998, 73, 2866.
Often the anode is composed of a transparent conducting oxide such as indium-tin oxide (ITO), the high work function of which is well matched in energy to the highest occupied molecular orbital (or valance band) in the light-emitting material. The cathode generally contains a low work function metal, for example, calcium or magnesium, such that the barrier for injection of electrons into the lowest unoccupied molecular orbital (or conduction band) of the organic layer, e.g., light-emitting material, is as small as possible. To provide more stability, these metals are typically alloyed with or covered by silver or gold.
The failure mechanism for such light-emitting devices is generally associated with degradation of the cathode. Tang et al., Appl. Phys. Lett., 1987, 51, 913-915. Other problems including oxide buildup and localized heating have also been attributed to this interface. See, for example, Choung et al., Appl. Phys. Lett., 1998, 72, 2689-2691; and Burroughs, et al., Nature, 1990, 347, 539-541.
Other approaches have been shown to improve performance at the cathode/organic interface. One method involves placing a very thin layer of an insulating salt such as LiF, MgO (Hung et al., Appl. Phys. Lett., 1997, 70, 152-154), MgF2 (Lee, Synth. Met., 1997, 91, 125-127) or Al2O3 (Tang et al., Appl. Phys. Lett., 1997, 71, 2560-2562) separating the organic material and an aluminum metal covering. Devices of this type are more efficient and have longer operating lifetime than previous models.
A series of low Φ conducting polymers have also been reported, one of which was used in conjunction with ITO as a cathode in an “inverted type” OLED. Bloom et al., J. Am. Chem. Soc., 2001, 123, 9436. The materials were composed of redox active substituted transition-metal diimine complexes, which as thin films were thermally polymerizable. Electrochemical reduction of the polymers yielded conductive films with work functions (which could be predicted from cyclic voltammetry of the monomers) from 3.7 to 3.0 eV. An OLED consisting of the layers gold/TPD/Alq3/polymer/ITO was reported where gold was the anode. (TPD is a commonly used organic hole transport material). While this type of device produces light under a moderate voltage bias, the performance was not optimal.
The latter strategy involves the use of a series of conducting polymers having a low work function, one of which was used as a cathode material for an OLED. Without being bound by any theory, it is believed that cathodes comprising a conductive organic material would be inherently more compatible with the light-emitting (i.e., luminescent) materials than the metals that are typically employed. There are inherent limitations of this system that make it of limited utility for the construction of devices, however, including the necessity of post-polymerization electrochemical processing of the polymers.
In view of the problems associated with current OLEDs, it is evident that there is a need for other low work function materials that can be used in a variety of electronic devices, such as OLEDs.